Numerous continuous level sensors have been devised to detect the level of liquid or solid particulate matter in bins or silos. Mechanical or electromechanical devices can be useful in liquids but are not reliable when dealing with particulate solids such as powders, grains, feed, and the like. Other sensors include ultrasonic, radar, and time, domain reflectometry. The ultrasonic sensor is popular because of its relative accuracy and simplicity of operation. The ultrasonic sensor is located at or near the top of a container and transmits ultrasonic energy through the air to the material surface. The time measured between the transmission and return of the ultrasonic energy is used to compute the distance from the sensor to the material level.
Ultrasonic sensors use a piezo-crystal which vibrates or “rings” to emit ultrasonic energy. Reflected ultrasound cannot be accurately measured until the piezo-crystal stops vibrating. Hence, ultrasonic sensors have a “dead zone” near the sensor where distance cannot be measured accurately. In addition, the ultrasonic sensor may indicate a false level under these conditions. In bins or silos where accurate estimates of material levels are essential to monitor inventory and/or consumption, ultrasonic sensors are not useful. For example, in bins or silos that are 12 to 30 feet in height, a single ultrasonic sensor would not be useful because the ultrasonic sensor could give a reading of any level if the material level is in the dead zone of the ultrasonic sensor. Also, ultrasonic sensors need to avoid contact with the material in the container in order to prevent contamination and malfunction.
One approach to solve this problem is the use of a second ultrasonic sensor which is a point level sensor for the detection of liquid. When the liquid level reaches a certain height the point level ultrasonic sensor provides a warning. However, the ultrasonic point level sensor has its own dead zone and is susceptible to producing erroneous readings and to potential contamination and malfunction. In addition, the use of combined ultrasonic sensors may be expensive, and there is no evidence that this combination is useful with non-liquid materials. What is needed, therefore, is an ultrasonic sensor system using a single continuous ultrasonic sensor, wherein the ultrasonic sensor dead zone can be accurately detected.